Every plastic recycling washing line produces wastewater — and what is in it determines whether your plant recycles water profitably or pays to discharge it. DAF (dissolved air flotation) is the standard technology for removing suspended solids (SS), fats, oils, and grease (FOG), and biochemical oxygen demand (BOD) from washing effluent. This guide explains how it works and how to specify it.
The Engineering Challenge: SS, FOG, and BOD
Plastic washing effluent is a difficult stream: dirt and paper fibers stay suspended, oils and grease float, and dissolved organics resist settling entirely. Conventional settling tanks handle only the easy fraction — the rest passes through to discharge or fouls recycled water. DAF attacks all three pollutant classes in one unit.
What SS, FOG, and BOD Actually Are — and Where a Washing Line Produces Them
- SS (suspended solids): dirt, sand, paper fibers, and fine plastic particles — generated at shredding, friction washing, and sink-float stages.
- FOG (fats, oils, grease): lubricants, food residues, and plasticizer leachate — buoyant, and nearly impossible to settle out.
- BOD (biochemical oxygen demand): dissolved organics from labels, adhesives, and food waste — consumes oxygen in receiving waters and drives discharge limits.
How DAF Technology Works
DAF dissolves air into water under pressure, then releases it into the treatment tank as microscopic bubbles. The bubbles attach to suspended particles and oil droplets, floating them to the surface as a sludge blanket that is continuously skimmed off. Clarified water exits from below.
The 4-Stage Process
- Coagulation: chemicals destabilize fine particles so they clump together.
- Flocculation: gentle mixing grows the clumps into floatable flocs.
- Flotation: micro-bubbles lift the flocs to the surface.
- Skimming: the sludge blanket is removed; clarified water discharges below.
Performance Comparison: DAF vs. Sedimentation
| Criterion | DAF | Sedimentation |
| Removes buoyant FOG | Yes — floats it out | No — oils pass through |
| Fine suspended solids | 90–99% removal | 50–70%, slow |
| Footprint | Compact | Large tanks, long retention |
| Response to load swings | Fast — minutes | Slow — hours |
| Chemical use | Coagulant + flocculant | Minimal |
What Removal Rates Depend On
Published removal rates (90–99% SS, 80–95% FOG) are achievable — but only when the unit is properly sized and operated. Key variables: influent solids concentration and particle size, FOG content, chemical type and dosing accuracy, air-to-solids ratio, and hydraulic loading rate. Dirty agricultural film effluent and clean bottle-wash effluent are different engineering problems; size and chemistry must match your actual stream.
How to Verify DAF Performance: Test Conditions That Make Numbers Honest
Demand performance data tied to stated test conditions: influent SS/FOG/BOD concentrations, flow rate, chemical dosing, and effluent quality — not headline percentages alone. Better yet, run a jar test or pilot trial on your own wastewater before committing: it reveals the right chemistry and realistic removal rates for your specific contaminants.
Sizing and Integrating DAF in a Washing Line
Size the DAF unit by hydraulic flow (m³/h) plus solids loading (kg/h) from your washing line — undersized units short-circuit and fail, oversized ones waste capital and chemicals. Integrate it as a side-stream or full-flow loop: collect wash water in an equalization tank, treat through DAF, and return clarified water to the non-final-rinse stages. A closed loop typically cuts freshwater consumption by 70–90%, which is where the payback lives.
Frequently Asked Questions
What is DAF in wastewater treatment?
DAF (dissolved air flotation) removes suspended solids, oils, and grease from wastewater by attaching microscopic air bubbles to contaminants, floating them to the surface for skimming. It outperforms sedimentation for light, buoyant pollutants common in plastic washing effluent.
Why does a plastic recycling washing line need DAF?
Washing plastic generates wastewater loaded with suspended solids (dirt, paper fibers), free oils and grease, and dissolved organics. DAF treats this stream so water can be recycled back to the line — cutting freshwater intake, discharge fees, and environmental compliance risk.
What do SS, FOG, and BOD mean?
SS (suspended solids) are visible particles; FOG (fats, oils, grease) are buoyant hydrocarbons that resist settling; BOD (biochemical oxygen demand) measures dissolved organics that consume oxygen as they decay. DAF targets SS and FOG directly and reduces BOD by removing the organics that carry it.
DAF vs. sedimentation — which is better for washing effluent?
DAF. Sedimentation relies on gravity settling, which is slow and fails on buoyant contaminants like oils and fine plastics. DAF actively floats them out in minutes with chemical assistance, in a fraction of the footprint.
What removal rates can a DAF unit achieve?
Well-operated DAF units typically remove 90–99% of suspended solids and 80–95% of FOG. Actual rates depend on influent characteristics, chemical dosing, and operating conditions — always verify with pilot or lab testing on your own wastewater.
How is a DAF unit sized for a washing line?
Size by hydraulic flow rate (m³/h) plus solids loading (kg/h). Undersized units short-circuit and fail; oversized ones waste capital and chemicals. Provide your supplier with flow data, influent analysis, and target effluent quality for proper sizing.
Can DAF effluent be reused in the washing line?
Yes — that is the primary economic case. Treated DAF effluent is typically clean enough for wash water reuse in non-final rinse stages, cutting freshwater consumption by 70–90% in closed-loop designs.
DAF is not an accessory — for any washing line above pilot scale, it is what makes water recycling economically viable. Size it to your real flow and solids load, verify performance on your own wastewater, and close the loop: the water you do not buy or discharge is margin you keep.
